It is desirable to have at least approximately the same density or supporting capability of the molding sand within a mold box, surrounding the molding pattern. In actual practice, friction along the surfaces of the mold box and the surfaces of the molding pattern, substantial differences in level of surfaces of the molding pattern, or narrow and deep contours of the molding pattern lead to different densities, different compaction, and hence different support capabilities of the sand. For example, if the pattern has surfaces which vary widely in level, it may happen that sand over elevated surfaces is excessively compacted whereas, in narrow and deep contours of the pattern, as well as at the surfaces facing the mold box, regions will be left in which the sand is only slightly compacted and substantially less so than at the regions of high pattern surfaces.
In general, two stages of compaction are used when the contours of the pattern are complex. Compaction of the back of the sand can be obtained by using a pressure plate with a multiplicity of press plungers. The press plungers are intended to compensate, automatically, for differences in compaction due to the pattern shape during the compaction process. German Patent Disclosure Document DE-OS 29 36 173, Geiger, describes an apparatus for compaction of sand in which the sand is first pre-compacted by a pressure plate and then, in a second step, press plungers, movable through the pressure plate, carry out final compaction. The press plungers are moved towards the pattern by being retained within a chamber which is subjected to a common pressure source, such as hydraulic or pneumatic pressure fluid. The press plungers penetrate into the back of the sand, more or less deep.
European Patent Publication 01 72 937, Fuchigami et al, describes carrying out compaction by a presser plate having a plurality of press plungers or squeeze heads therein; subsequently, and with the mold box raised, a final compaction is obtained by pressing the entire assembly together.
Multiple press plunger presser plate are also known in which press plungers adjacent the circumference of the mold box are subjected to a higher pressure level than press plungers which are more centrally located, when the filled mold box is moved upwardly against the press plunger by a suitable lifting table or the like. This results in somewhat better matching of the press plungers to the contour of the pattern during the plunger stroke; no account is taken, however, of differences in shape of the pattern itself and differences and compaction or density of the sand will result, taken across the pattern. It is, thus, usually necessary to combine such a system with compaction by shaking or vibration.